1. Field of the Invention
This invention relates to a power supply for welding machines using a consumable or non-consumable electrode, and more particularly to an inverter type power supply system.
2. Description of the Prior Art
One example of a conventional power source for DC welding involves a system arranged to drop the line voltage of a three-phase AC source to about 40 to 85 V by means of a transformer and to phase control the dropped voltage by a thyrister for controlling its voltage or current and for rectification, supplying a predetermined DC current to a welding electrode through a reactor. The welding power supply of this sort has a drawback in that it is very heavy, for example, weighing about 160 kg in the case of a power supply of 500 A, due to the use of a transformer and a reactor which are heavy and large in size, and therefore is difficult to move or handle. Besides, the phase control of the line voltage generates ripple voltages of long periods at a low output voltage, destabilizing the welding current and voltage to invite welding defects such as spattering, defective bead appearance and insufficient penetration.
In order to eliminate the above-mentioned difficulties, there has been developed a welding power supply using a high frequency type inverter, in which the line AC power is rectified and converted into alternate current of a higher frequency (e.g., 2 kHz) by an inverter, the converted voltage (to 40 or 85 V) is dropped by a transformer and rectified through a rectification circuit for supplying DC power of a predetermined voltage to a welding electrode through a reactor.
The transformer and reactor in the high frequency inverter type power supplies are smaller in size and weight, so that it is possible to reduce the size and weight of the power supply system as a whole while minimizing the ripple component of the welding DC voltage and current to permit relatively stable welding operations.
As illustrated particularly in FIG. 1, the inverter circuit which is employed in such a welding power supply generally has transistors 11 to 14 connected bridge-like between output terminals of a rectifier circuit 7 with diodes 1 to 6 and turned on and off by the output signal of a drive circuit 20 in the manner as described hereinafter, thereby to produce an alternate current in the output transformer 21.
Namely, when the transistors 11 and 13 are simultaneously turned on, current flows through the output transformer 21 in the direction of arrow A. On the other hand, if the transistors 11 and 13 are simultaneously turned off by the drive circuit 20, while instead turning on the transistors 12 and 14 simultaneously, current flows through the output transformer 21 in the direction of arrow B. This on-off operation of the transistors is repeated at a frequency of 2 KHz, for example, to supply alternate waves to the output transformer 21.
The output of the transformer 21 is rectified through a rectifier circuit 22 and applied to a welding electrode 24 through a reactor 23 to generate an arc across the welding electrode 24 and a base metal 24' thereby to perform a predetermined welding operation.
The welding voltage and welding current are controlled by varying the conduction period t of the transistors 11 to 14 as shown particularly in FIGS. 2a, 2b and 2c depicting output waveforms as obtained by using a resistance load without a DC reactor, namely, at FIG. 2(a) a waveform at the maximum output, at FIG. 2(b) a waveform at an output level of 1/2 the maximum output, and at FIG. 2(c) a waveform at an output level of 1/5 the maximum output.
As seen from FIG. 2(c), the pulse width t is narrowed in a low welding current range, inviting deteriorations in the welding characteristics due to an increased ripple content in the output voltage. In this regard, although the ripple content of the output voltage can be minimized by increasing the reactance of the reactor 23, it is desired to minimize the reactance of the reactor 23 as much as possible since a higher impedance of the reactor 23 will delay response and lower efficiency.
The above-mentioned inverter type power supply has another problem in that a ripple component attributable to the characteristics of the transistors of the inverter circuit appears in the range of low welding current and voltage. In this connection, reference is had to FIGS. 3a, 3b and 3c in which there are shown at (I) to (III) one-period waveforms at high, medium and low load currents, respectively. Shown at FIG. 3a is the output waveform of the rectifier circuit 22, at FIG. 3b the base input of the transistors 11 and 13, and at FIG. 3c the base input of transistors 12 and 14.
In FIGS. 3a, 3b and 3c, t.sub.on indicates the turn-on time of the transistors 11 to 14, t.sub.off their turn-off time, and t.sub.stg their storage time.
Referring to the circuit diagram of FIG. 1, even if high speed switching transistors are used for the transistors 11 to 14, they have a turn-on time t.sub.on of 1 .mu.s, a turn-off time t.sub.off of 1 .mu.s and a storage time t.sub.stg of 2 .mu.s. Therefore, in a case where the inverter frequency is 50 KHz and used in a range lower than 1/3 of the maximum output, the output becomes unstable in a range smaller than 1/3 of the maximum output. If the output is lower than 1/3, there will be produced an output of 1/3 at every two or three periods, with an output of 1/6 to 1/10 in average. This means increases of the ripple current and voltage, which will lead to destabilization of the welding current, increased spattering, defective bead appearance and insufficient penetration. The ripple component can be reduced by increasing the reactance of the reactor but it will lower the response and efficiency as mentioned hereinbefore.
The rates of t.sub.on, t.sub.off and t.sub.stg at 1/2 periods can be reduced by lowering the inverter frequency, thereby to stabilize the low output current and voltage. However, it will increase the weight of the transformer and that of the reactor contrary to the initial objective of minimizing the transformer and reactor size and weight by increasing the inverter frequency.
In addition, the welding power supply of this sort is required to be capable of stabilizing the welding characteristics over large and small current ranges. For instance, a welding power source of 500 A should not show a material drop in welding characteristics at a 1/3 output level, namely, at 170 A.
On the other hand, in dip transfer welding in which short circuiting and arcing alternately take place, there occur large differences in voltage and current between a short circuit period and an arcing period. Therefore, in designing a power supply for dip transfer welding, it has been the general practice to afford a large capacity far higher than an average output, and thus to employ component parts of large capacities, resulting in a power supply system which is increased in weight and size as well as in cost.
Further, in order to suppress spattering effectively in the dip transfer welding, the power supply is required to be able to control the current and voltage with high response and at a suitable time point in a final stage of each short circuit period of large current since otherwise intense spattering occurs by explosive rupturing of the short circuit.